β-PEA and a DAD1R antagonist, SCH23390, were purchased from Sigma-Aldrich (St. Louis, MO, USA). β-PEA and SCH23390 were dissolved in 0.9% physiological saline. The intraperitoneal (i.p.) injection for SCH23390 was determined based on a previous study [63 (link)]. For self-administration studies, a working solution of saline or β-PEA was filtered through a syringe-mounted 0.22-µm Minisart® Syringe Filter (Sartorius Stedim Biotech, Goettingen, Germany) immediately before use. All drug solutions were prepared immediately prior to the beginning of each experiment.
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